The wind turbine comprises a tower on which a nacelle is mounted by a yaw turning system. The nacelle supports a hub to which the blades are connected. In a conventional wind turbine the hub turns a main shaft which in turn drives a gear of a gearbox connected to a generator. In such wind turbines, the hub and main shaft rotate with a comparably low speed, whereas the generator rotates with a comparably high speed in order to achieve the network frequency, i.e. 50 Hz or 60 Hz, as required by the power networks to which the wind turbine is connected.
In order to reduce the production and maintenance costs associated with an additional gearbox, direct drive wind turbines are being introduced into wind parks. The principle of operation of a direct-drive wind turbine is disclosed in EP 2 157 314 A1, which is incorporated herein by reference, and in “Windkraftanlagen”, Robert Gasch and Jochen Dreher, E W S., 6th edition 2010, Vieweg+Täubner.
In a direct drive wind turbine, the generator rotates with the speed of the rotor, since the rotor is directly structurally coupled to the hub, without any gearbox. For control of the wind turbine it is essential to keep the speed of the hub within an acceptable range. Therefore, the rotational speed of the hub must be monitored closely. To this end, a direct-drive wind turbine generally comprises a rotation sensor for monitoring the rotational velocity of the hub.
In a state of the art direct drive wind turbine described in U.S. Pat. No. 7,431,567 B1, the rotation sensor is suspended between the hub and the stator of the generator. Since there is no shaft in such a wind turbine, the rotation sensor is mounted on supports to keep the rotation sensor in place. Because the rotation sensor is large and is suspended in the cavity of the stator, access to components such as the hub is partially blocked, so that maintenance is made more difficult and time-consuming. For example, parts of the rotation sensor may first have to be dismantled in order to gain access to a cavity of the hub. Furthermore, this type of rotation sensor can only provide a low resolution of measurement, and also results in inaccuracies owing to movement between the hub and the stator. In order to accurately measure the rotational speed of the hub to be able to control the wind turbine, a minimum resolution of at least 300 steps or samples per revolution of the hub is necessary.